Madhu,
String_Split function will help you to create a list of text, based on a delimiter.
For example if you have a string like:
1,2,3,4
And use this string in String_Split with comma as a delimiter it will return a list with:
1
2
3
4
I think this is not your user case here.
What I think you need is Substr() function, see:
With it you will be able to get for example just the 2 first digits and after the rest of your string.
You can then use it like this for example:

Substr(Text,0,2) //This would be the first 2 digits, for example extern code
+
Substr(Text,2,Length(Text)) //This would be the rest of your telephone
Hope it helps,
Cheers and Regards,
RR :)